The IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). Whilst SIP was created as a user-to-user protocol, the IMS introduces additional functionality for e.g. subscription handling, security and charging to allow operators and service providers to control user access to services and to charge users accordingly.
FIG. 1 is a network diagram illustrating schematically how the IMS fits into mobile network architecture in the case of a General Packet Radio Service (GPRS) access network. The IMS 3 includes a core network 3a, and a Service Network 3b. Call/Session Control Functions (CSCFs) 4 operate as SIP proxies within an IMS core network 3a. User Equipment (UE) 2 accesses the IMS 3 via an IP Connectivity Access Network (IP-CAN) and gateway nodes in a connectivity layer 1. A Home Subscriber Server HSS 8 keeps a log of users' subscription profiles that define the services that the user has subscribed to. After registering, the user is able to establish a communication session with other peers, making use of the multimedia capabilities of the IMS. The 3GPP architecture defines a number of types of CSCFs including: a Proxy CSCF (P-CSCF) which is the first point of contact for the user within the IMS core network 3a; a Serving CSCF (S-CSCF) controls the provision of services to the users in accordance with the user's subscription; an Interrogating CSCF (I-CSCF) whose role is to identify the correct S-CSCF, based on the user's subscription profile which it obtains from the user's HSS 8, and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.
IMS Emergency Services specified in the 3rd Generation partnership project (3GPP) Technical Specification (TS), 3GPP TS 23.167 define the architecture and mechanisms required for a user to be able to establish IMS emergency sessions towards Emergency Centers and Public Safety Answering Points (PSAPs). This architecture includes an Emergency CSCF (E-CSCF).
The related detailed mechanisms are defined in TS 24.229 including the means by which a UE indicates that an IMS Registration is triggered for the purpose of establishing an emergency session; and the means by which an IMS emergency session is established. Additionally, TS 24.229 defines the means by which the IMS serving network detects that an IMS session is established for the emergency service. The P-CSCF must be prepared to detect an emergency session even if the UE does not use the emergency service URN.
When an emergency call is made, it may often be that there are friends, family members or other trusted associates of the user who are nearby and could be notified or contacted to assist the emergency services. The present disclosure provides mechanisms by which this can be achieved.